[Session restore] Add MRU logic to loading of background pages.

components/sessions/session_types.h

Index: components/sessions/session_types.h
diff --git a/components/sessions/session_types.h b/components/sessions/session_types.h
index a7f035891ba5c6e2db6bb70eb0c4f1f238886393..c71445e4800c2d6b57faba9668b4f46db0b38095 100644
--- a/components/sessions/session_types.h
+++ b/components/sessions/session_types.h
@@ -97,6 +97,11 @@ struct SESSIONS_EXPORT SessionTab {
   // Timestamp for when this tab was last modified.
   base::Time timestamp;
 
+  // Timestamp for when this tab was last activated. As these use TimeTicks,
+  // they should not be compared with one another, unless it's within the same
+  // chrome session.
+  base::TimeTicks last_active_time;
+
   std::vector<sessions::SerializedNavigationEntry> navigations;
 
   // For reassociating sessionStorage.